Numerous operations are performed on oil and gas wells which require large volumes of nitrogen gas or other cryogenic fluids. These operations may be performed on both onshore and offshore wells. Such operations include foam fracturing operations, acidizing services, jetting down the tubing or down the tubing-casing annulus, nitrogen cushions for drill stem testing, pressure testing, insulation of the tubing-casing annulus to prevent such problems as paraffin precipitation, jetting with proppant for perforating and cutting operations, reduction of density of well workover fluids, displacement of well fluid from tubing during gun perforation operations to prevent excess hydrostatic pressure in the hole from pushing perforation debris into the formation, placing corrosion inhibitors by misting the inhibitor with nitrogen, extinguishing well fires, and other operations. Other operations that require cryogenic fluids at an ambient temperature include pipeline and vessel purging operations and refinery operations such as, recharging catalysts.
Nitrogen is typically stored in its liquid state because of the volume used however, liquid nitrogen will damage most carbon steel pipes used in oil and gas wells. Thus, various heating systems have been developed to raise the nitrogen to an ambient temperature. Typically, 185 BTUs per pound of nitrogen are required to heat the nitrogen to an ambient temperatures of 70.degree. F.
One particular such operation is the fracturing of a subsurface formation of the well by pumping a fluid under very high pressure into the formation. The fracturing fluid which is pumped into the well often comprises a foamed gel which is produced by the use of nitrogen gas. The nitrogen for the foam fracturing operation is generally stored in a fluid form at temperatures of approximately -320.degree. F.
At pressures encountered in these foam fracturing operations, the nitrogen changes state from a liquid to a gas at approximately -200.degree. F. It is, therefore, desirable to heat up the nitrogen gas to a superheated state so that the foam fracturing fluid being pumped down the well will be at an essentially ambient temperature. This is because of the numerous adverse affects upon mechanical equipment of very low temperature which would otherwise be presented by the nitrogen foam.
With regard to land based wells, the nitrogen heating equipment generally includes open flame heaters. A problem is however, presented when performing foam fracturing operations on offshore wells. For safety and environmental reasons, open flames are generally not allowed on an offshore drilling platform. Therefore, it is necessary to provide a heater for the nitrogen which does not have an open flame.
Such flameless nitrogen heaters have previously been provided by utilizing the heat generated by an internal combustion engine and mechanical components driven thereby to heat a coolant fluid which transferred that heat to the nitrogen through a coolant fluid-to-nitrogen heat exchanger.
Numerous problems are encountered with prior art devices mainly because of the use of air as a heat transfer medium. Air is a notoriously poor heat transfer medium as compared to a liquid and the use of ambient air causes the system to be dependent upon ambient air conditions for proper operation. Additionally, due to the large bulky nature of the plenum chamber required for the use of air as a heat transfer medium, the air systems are typically very bulky and heavy. Therefore, there is a need for a flameless nitrogen unit that is compact in size, efficient in the heat transfer process, and economical.